In prior art, e.g. document JP 2005087876 discloses a hydrofoil impeller or producing fluid flow in axial direction relative to a shaft rotating around its central axis in an agitated tank. The impeller comprises a central hub which is connected to the shaft. The central hub is in the form of a flat plate with a uniform thickness and being perpendicular to the central axis. The central hub has three groups of first bolt holes arranged to form a pattern. Three equally-spaced blades extend radially outwardly from the central hub. Each blade has a root portion. The blade is in a form of a flat plate with a uniform thickness. The root portion has a group of second bolt holes arranged in a corresponding pattern in relation to the pattern of the first bolt holes, so that the group of second bolt holes can be aligned with the group of first bolt holes and bolts can be placed through the first and second bolt holes to form bolted joints. Each blade further comprises a straight leading edge, a trailing edge and a tip edge.
The applicant of the present application has previously designed a blade of an axial flow impeller and an axial flow impeller, disclosed in WO 2013/124539 A1, the design and dimensioning of the blade having excellent characteristics in terms of flow pattern, low energy consumption, high pumping capacity, strong axial flow with a small power consumption and low shear, high pumping efficiency, scalability and low fabrication costs. However, the presented blade design is suitable for blades which are connected to the central hub by welding. Now, there has existed a need to develop a blade and an impeller which can be constructed without any welding, so that such a structure can provide for lower manufacturing costs and enhanced fatigue strength. Therefore, the attachment of the blades to the central hub by bolted joints is a desirable approach. However, the bolted joints need redesigning of the central hub, the pattern of the bolted joint attachment and the form of the blade so that an equally good performance can be achieved compared to the impeller and blade design presented in WO 2013/124539 A1.